1. Field of the Invention
This invention relates to the copying of a photomask pattern onto a workpiece, particularly, a semiconductor substrate, and also to an alignment mark used to align the substrate and the photomask relative to each other prior to the projection of the photomask pattern onto the substrate.
2. Description of the Prior Art
A projection exposure apparatus is often used when a pattern on a reticle or a photomask is to be copied onto a semiconductor substrate to which photoresist is applied. In this exposure apparatus, a pattern depicted on a reticle or a photomask is projected through a projection optical system, for example, a 1/5 reduction projection lens, and the projected image is aligned with a photosensitive substrate (a wafer or the like) on which a pattern has been pre-formed and is superposedly printed on the substrate, whereby the lithography of a desired semiconductor device is effected. In this case, if the projected image of the pattern of the photomask is not accurately imaged on the wafer, a blurred pattern will be formed on the wafer, thus causing a problem of so-called unsatisfactory resolution.
Further, if the projected image and the pattern formed on the wafer are not accurately aligned, there will be caused a problem that the characteristic of the semiconductor device manufactured becomes insufficient or that in an extreme case, the operation of the semiconductor device itself becomes impossible. To solve such problem, it is required that the projected image be accurately focused to the surface of the wafer and the photomask and the wafer be accurately aligned. For this alignment, in recent years, a so-called through-the-lens (TTL) type alignment apparatus has come to be used in which an alignment mark formed on a wafer through a photomask and a projection optical system is observed with the alignment mark on the mask. In this TTL system, the mark on the wafer and the mark on the mask are detected at a position whereat they are to be projected and exposed through the projection optical system and therefore, high alignment accuracy can be expected. This TTL type alignment apparatus is disclosed, for example, in U.S. Pat. No. 4,362,389 issued on Dec. 7, 1982. In this patent, there is disclosed an apparatus in which, on a predetermined chip formed as a matrix on a wafer, a linear wafer alignment mark whose extensions intersect so as to be substantially orthogonal to one another, and which is arranged radially outwardly from the center of the chip, and another wafer alignment mark are separately formed and a square or linear reticle alignment mark is formed at a position of a photomask corresponding to the position of said linear wafer alignment mark. A projection lens is disposed between the wafer and the mask and further, an X-Y table on which the wafer is placed is step-advanced with each chip spacing. The wafer alignment mark and the reticle alignment mark superposed one upon the other at at least two locations by the projection lens are illuminated, and the optical images thereof are picked up by an image pickup device. From the resultant image signals, the relative displacements of the two marks (with respect to the X-axis direction and the Y-axis direction) are detected, and the wafer and the mask are moved relative to each other so that each detected displacement may become null.
The wafer alignment mark is a linear projection or a linear groove formed on a semiconductor substrate, and the alignment of the wafer and the mask is usually effected with photoresist being applied to the wafer. In the use of the alignment apparatus as described above, the wafer alignment mark is illuminated by a monochromatic light (of the same wavelength as the exposure light) through the photomask and the projection lens, and the alignment mark of the wafer and the alignment mark of the mask are superposedly imaged on a predetermined conjugate surface. These images are picked up by the image pickup device and converted into image signals, which are then subjected to predetermined signal processing to thereby align the mask and the wafer. In the portion of the wafer corresponding to the alignment mark the illuminating light partly reflected by the surface of the photoresist film and the illuminating light reflected by the surface of the semiconductor substrate interfere with each other.
When the optical refractive index of the photoresist is n and the thickness of the photoresist film is d, the optical distance D between the surface of the photoresist and the surface of the semiconductor substrate can be expressed as D=n.d. Accordingly, the condition under which the interference phenomenon occurs is n.d=.lambda./2, where .lambda. is the wavelength of the monochromatic illuminating light. That is, d=.lambda./2n. If, for example, a monochromatic light of .lambda.=436 is used for the illumination, and n.div.1.67, d.div.130 nm. Thus, when the thickness of the photoresist film is integer times 130 nm, interference fringes occur.
The alignment mark of a wafer is generally comprised of a projection or a groove formed on the surface of a semiconductor substrate via the etching process. The alignment mark is discriminated by a difference level formed between this projection or groove and its surroundings. Photoresist uniformly applied to the surface of the semiconductor substrate including the alignment mark is liquid having suitable viscosity and surface tension in the step during which it is applied to the substrate and therefore, the thickness of the photoresist itself is small in the portion thereof corresponding to the projection on the substrate and great in the portion thereof corresponding to the groove. That is, the thickness of the photoresist film differs greatly.
Near the difference level at the boundary of the alignment mark, the thickness of the photoresist film varies gently so that it becomes gradually small or great away from the difference level. If a portion of the thickness which satisfies the above-described interference condition exists in this gentle variation, the interference phenomenon will occur in such portion and as a result, interference fringes will occur along the difference level edge.
The position of the alignment mark of the wafer is recognized by detecting, for example, the rising portion and the falling portion of the image signal corresponding to the contrast of the projected image by the projection lens and detecting the middle of the rising portion and the falling portion. Thus, if interference fringes occur, the position of the alignment mark of the wafer will be erroneously recognized.